To conveniently support an electrical wiring device, such as a switch or a receptable, an electrical lighting fixture, or simply a cover plate, an electrical outlet or wiring box is mounted in an opening made in a wall or ceiling and the electrical apparatus or cover plate is fastened to the box by means of mounting scres. Conventional electrical outlet boxes are formed from plastic or metal, e.g., aluminum, and typically have performed internally threaded bores for receiving and threadedly engaging the mounting screws, or have spring clips or other inserts to fasten the mounting screws to the electrical box as the screw passes through a bore formed in each of the box bosses. The use of performed threads or inserts in the bore is too expensive.
Examples of these prior art devices regarding outlet boxes as well as other screw retaining assemblies are disclosed in the following U.S. Pat. Nos.: Re. 29,752 to Jaconette, Jr.; 3,263,949 to Conrad; 3,876,821 to Pringle; 3,967,049 to Brandt; 4,025,144 to Thibeault; 4,130,335 to Kinney; 4,295,003 to Borja et al; and 4,355,198 to Gartland, Jr.
Commonly assigned, U.S. Pat. No. 4,580,689, entitled "Electrical Outlet Box With Integral Screw Aligning, Engaging And Retaining Boss" and issued Apr. 8, 1986, to William E. Slater discloses a nonmetallic box for supporting a device by a screw. The box comprises a boss with a through bore. The through bore has, in sequence from the boss front face, a frustoconical surface, a first cylindrical surface having a diameter slightly larger than the crest diameter of the screw threads, and a second self-tapping cylindrical surface having a diameter slightly smaller than the crest diameter of the screw threads and slightly larger than the root diameter of the screw threads. The subject matter of this patent is hereby incorporated by reference.
Commonly assigned, copending U.S. patent application Ser. No. 808,817, entitled "Electrical Outlet Box with Polygonal Mounting Bore" and filed Dec. 13, 1985, to Thomas E. Lewis, discloses a nonmetallic outlet box for supporting an electrical device by a threaded screw having screw threads with a crest diameter and a root diameter. The outlet box comprises a boss integrally formed with the box and having a front face. A straight, laterally closed bore extends inwardly of the boss from its front face for receiving the screw. This bore has a substantially regular polygonal configuration, i.e., the polygon has equal sides and included angles. The polygonal configuration has an even number of sides, preferably in the form of a hexagon, and has a diameter across its corners slightly larger than the crest diameter of the screw threads, and a diameter across its flat sides slightly smaller than the crest diameter of the screw threads and slightly larger than the root diameter of the screw threads.
This arrangement maximizes the ratio of stripping torque to running or threading torque by only tapping threads in parts of the flat sides of the polygon. The subject matter of this copending patent application is hereby incorporated by reference.
However, the laterally closed bore presents potential problems in manufacture and use with metallic, particularly aluminum, boxes. Molding the cross-sectionally hexagonal bores requires long and slender projections on the mold. For aluminum boxes, as compared to plastic boxes, the long and slender projections are more likely to break because of the higher temperature cycles, pressures, density and abrasive characteristics of the aluminum. The torque necessary for forming the threads in aluminum boxes with the mounting screws is greater than that for plastic boxes.
Accordingly, there is a continuing need for improvement in rapidly, easily and economically mounting various types of electrical devices or covers to metallic and nonmetallic outlet boxes, and for forming those outlet boxes.